1. Field of the Invention
This invention relates to a stretch-thinned article having dimensional stability over time and at elevated temperatures and a method for producing such a stretch-thinned article. More particularly, this invention relates to stretch-thinned films and laminates which can be used as components in durable and disposable articles. Disposable articles may include personal care articles such as diapers, incontinence wear, training pants, feminine care articles, etc. Components of such articles include side panels, outer covers, waist bands, supporting straps, absorbent wraps, stretchable liners and the like, etc. Also, articles such as wound dressings, wipes, towels, napkins, protective apparel, etc. can contain the films and laminates of this invention.
2. Description of Prior Art
The development of polymer films and processes for producing these films has continued to increase over the years. Such polymer films have numerous uses such as personal care articles such as diapers, incontinence wear, training pants and the like as well as articles such as wound dressings, wipes, protective apparel and the like. Depending upon the application, these films may be microporous so as to lend breathability to such articles and/or they may be laminated to other materials such as nonwovens so as to provide an effective barrier to the passage of body exudates while exhibiting good aesthetic and tactile properties, such as hand and feel.
One technique employed to achieve a satisfactory low-cost film has been to use films of increasingly lesser gauge or thickness. In addition to being lower in cost due to the reduced gauge, thinner films have increased softness. Such low gauge films are produced by drawing or stretching whereby the molecular structure of the polymer molecules is oriented within the film in the direction of stretching, thereby increasing the strength of the film in the stretched direction. However, one problem associated with such films is their general lack of dimensional stability over time, rendering them unsatisfactory for use in those applications requiring a high degree of dimensional stability. For example, for applications in which the films are cut to a specific size and then, sometime later, disposed in a location where the exact dimensions of the cut film must be met, shrinkage of the film prevents that cut film from being used. In addition, for films which are microporous, shrinkage of the films may reduce the size of the pores, thereby reducing the effectiveness of the pores as a means for passing water vapor and the like through the film and away from the wearer of articles comprising such films. Furthermore, dimensional stability of many stretch-thinned films is negatively impacted by exposure to elevated temperatures and, as thinner films are employed, the tendency of the films to break increases.
Accordingly, there is a need for a method for producing stretch-thinned films which address the issues set forth hereinabove and, in particular, stretch-thinned films having dimensional stability both over time and at elevated temperatures.
It is one object of this invention to provide a method for producing a stretch-thinned article, such as a film or fiber, which is dimensionally stable overtime and at elevated temperatures. That is, the article, after having been stretch-thinned retains its shape, even at elevated temperatures.
It is another object of this invention to provide a method for producing breathable films which are dimensionally stable both over time and at elevated temperatures.
These and other objects of this invention are addressed by a method for producing a stretch-thinned elastic article having dimensional stability over time and at elevated temperatures in which a thermoplastic block copolymer is melt-processed into an article, such as a film or fiber, and raised to a temperature equal to or greater than the glass transition temperature, Tg, of the hard phase of the thermoplastic block copolymer. The article is then stretch-thinned at the elevated temperature to a desired percentage stretch, forming a stretch-thinned article. The stretch-thinned article is then rapidly cooled to a temperature below the glass transition temperature of the hard phase of the thermoplastic block copolymer resulting in a dimensionally stable stretch-thinned article.
Depending upon the use of the article, the stretch-thinned article can be imparted with different characteristics directed for such usage. In accordance with one embodiment of this invention in which the stretch-thinned article is a film, the thermoplastic block copolymer is loaded with a filler, such as calcium carbonate particles, which produces a plurality of micropores in the film during stretch-thinning, resulting in a breathable stretch-thinned film.